This invention relates to reflex cameras with built-in exposure metering apparatus, and, more particularly, to an improvement in a light meter arrangement for a single lens reflex camera. Still more particularly, it relates to the provision of a glass member positioned in contacting relation behind the exit surface of a penta-prism of the camera so that a light value sensing means may be positioned to detect those portions of the light radiated from an object image formed on a focusing screen under the penta-prism which emanate from an upper portion of the exit surface of the penta-prism both after successive reflection from the two roof surfaces and one of the front reflection surfaces and after successive reflection from the two roof surfaces, one of the front reflection surfaces and again one of the roof surfaces.
It is known in the art to utilize that portion of the light radiated from the image which, after successive reflection from the two roof surfaces and one of the front reflection surfaces, is directed to one of the roof surfaces in measuring the light by use of light value sensing means positioned adjacent the roof surface in a photometric light path passing through a light-permeable portion thereof. This arrangement has a drawback in that the light-permeable portion must be located outside that portion of the roof surface from which a view-finder image forming light beam is directed to an eye-piece behind the exit surface of the penta-prism. Otherwise the entire image can not be viewed by the photographer looking through the eye-piece. In order to avoid introduction of any bad influence into the view field of the finder, therefore, it is required that the penta roof surfaces be of relatively large areas. Conversely, with a penta-prism of which the size of minimized under the condition that the light radiated from the image is only directed to the eye-piece, it is impossible to utilize any portion of the light reflected from the front reflection surface in measuring the light without introducing any bad influence to the view-field of the finder.
Also, it is known to measure the light from the image at two portions laterally spaced from each other by use of two light value sensing means positioned respectively on the right and left sides of the eye-piece. In making an average exposure determination by taking into account the light values measured at upper and lower portions of the image, however, it is impossible to position two light value sensing means respectively on the upper and lower sides of the eye-piece without unduly increasing the area of the exit surface of the penta-prism which in turn causes an objectionable increase in the bulk and weight of the penta-prism.